It may not be Hollywood's most enduring mystery, but the truth can finally be revealed over why Elvis Presley's trademark quiff turned from near-blond to jet black in the 1956 classic Love Me Tender - he was allowed to do his own hair.

The revelation is one of many Tinseltown secrets in a treasure trove of documents belonging to 20th Century Fox that are to be auctioned in New York next month to benefit an actors' charity.

Alongside details of Elvis's follicular faux pas are letters chronicling Judy Garland's slide into alcoholism, contracts showing the bumper pay packets of Marlon Brando and Cary Grant, and a 1946 memo declaring that Norma Jean Dougherty would henceforth be known as Marilyn Monroe.

"These papers are so cool that, as a fan of that history, I will have to restrain myself from bidding," said Tom Rothman, chairman of Fox Filmed Entertainment, in an interview with Variety magazine.

"Fox's heritage reflects Hollywood itself and our archives are a Tut's tomb of movie history. This donation is intended to get the past out of file cabinets into the hands of film lovers, and let it serve the present through the Motion Picture and Television Fund.

"We also hope this auction might inspire other studios to follow suit."

The documents, many signed by the actors, provide a tantalising glimpse into the professional lives of movie stars from Hollywood's golden era.

The contract between Presley and Fox for Love Me Tender gave the actor and singer unprecedented permission to wear his hair how he chose, but the unfortunate colour change later came to be seen as one of the film's most glaring continuity errors.

Garland's signature appears on a contract for the 1967 film Valley of the Dolls, for which she was cast in the supporting role of Helen Lawson. It was widely believed that she backed out of the film, taking her expensive sequinned trouser suit with her, but another letter in the collection confirms her dismissal for repeatedly turning up for filming late and inebriated. She died two years later from a drug overdose.

Brando, who received an Oscar nomination for the 1952 movie Viva Zapata, earned $125,000 (£63,000) for the 12-week shoot in Texas, according to another contract that will go under the hammer at the Swann Auction Galleries in Manhattan on January 25.

Grant, meanwhile, pocketed $100,000 for his appearance in the 1949 comedy I Was a Male War Bride plus 10% of the gross receipts if they reached $1m. The huge financial reward, and the perk of his own dressing-room phone, presumably provided some relief for Grant, who contracted hepatitis during filming in a freezing German winter.

Some of the other big-name contracts up for sale include two that highlight Humphrey Bogart's rise to stardom. His first studio agreement in 1930 gave him $400 a week for Up the River, but by 1951, when he signed up to play newspaper editor Ed Hutcheson in Deadline USA, his weekly salary was $17,500.

Earlier film stars such as Laurel and Hardy are also represented. Their first contracts stipulated that they had to supply their own costumes. As Fox's stable of stars grew, the studio did start providing clothes, though according to the documents some were still required to fork out for their own underwear.

Nicholas Lowry, the president of Swann Galleries, was unable to estimate the value of the documents, but said they would be worth a fortune to private collectors.

Ken Scherer, chief executive of the Motion Picture & Television Fund's foundation, which supports retired actors and their families, said the sale would provide a massive windfall.

"You can buy costumes and memorabilia on eBay, but these documents are something else," he told Variety. "Who knows the value of these items, many of which are pretty special and unique."